The present invention is related to bending of pipes, and more particularly to a method and an apparatus which allow metal pipes to be bent at a radius of curvature as small as possible relative to the pipe diameter.
As one prior art means for bending pipes, there is known a method in which a pipe 1 is pressed against the outer periphery of a formed roll 2 and turned as shown in FIG. 4. More specifically, one end of the pipe is clamped between a fixed clamp member 5a provided at one portion of the formed roll 2 and a movable clamp member 5b provided at one portion of a rotary base 3 which in turn is rotated about the rotary shaft 9 of said formed roll 2. By turning the rotary base 3 along the outer periphery of the formed roll 2, the pipe 1 is bent at the radius of curvature R of the formed roll 2.
A retaining base 4 for retaining the pipe 1 linearly in place is positioned at the rear of the rotary base 3. A mandrel 6 is inserted and held at a given position by means of a mandrel bar 7 inside the pipe 1 which in turn is retained by the retaining base 4. As shown in FIG. 4, the mandrel 6 is disposed at a position such that the tip 6a thereof extends slightly beyond a phantom line 10 which extends from the rotary shaft 9 of the roll 2 and intersects the axial line 8 of the pipe 6a at a normal angle. When one end of the pipe 1 is pulled and bent along the outer periphery of the roll 2 as the rotary base 3 is turned, the pipe surface on the outer side in respect of the roll 2 is pulled and tends to become flattened at a position slightly beyond the retaining base 4 as shown by the dash-and-dot line at 11. The tip end 6a of the mandrel 6 acts to prevent such flattening of the pipe.
As is clear from FIG. 4, the mandrel 6 is held firmly in place at a given position by means of the mandrel bar 7 because if the tip end 6a thereof extends unnecessarily beyond the phantom line 10, the mandrel 6 would get in the way and hamper the bending of the pipe 1.
The mandrel 6 disposed inside the pipe 1 helps prevent flattening of the pipe 1 at a location slightly beyond the retaining base 4 when the pipe 1 is bent by the turning of the rotary base 3. However, despite the presence of the mandrel within the pipe, it was still impossible in the prior art to bend pipes at curvatures that are small relative to the pipe diameter. As shown in FIG. 4, it is generally known that when the pipe diameter D and the radius of curvature R of the formed roll 2 hold the relation R=3D at least, pipes can bent without deformation to the curve. In other words, when the relation between the pipe diameter and the radius of curvature of the formed roll is below R=3D, presence of the mandrel 6 inside the pipe 1 cannot prevent flattening 11 of the pipe 1 as indicated by the dash-and-dot line at a portion ahead the mandrel 6.